Weight loss significantly improves physical health but effects on mental health are less straightforward, finds new UCL research funded by Cancer Research UK.

In a study of 1,979 overweight and obese adults in the UK, people who lost 5% or more of their initial body weight over four years showed significant changes in markers of physical health, but were more likely to report depressed mood than those who stayed within 5% of their original weight.

The research, published in PLOS ONE, highlights the need to consider mental health alongside physical health when losing weight. Clinical trials of weight loss have been shown to improve participants’ mood, but this could be a result of the supportive environment rather than the weight loss itself, as the effects are seen very early on in treatment and are not related to the extent of weight loss.

It’s important to note this new result does not mean that weight loss necessarily causes depression directly, as depression and weight loss may share a common cause. However, it shows that weight loss outside the clinical trial setting cannot be assumed to improve mood and raises questions about the psychological impact of weight loss.

The data came from the English Longitudinal Study of Ageing, a UK study of adults aged 50 or older, and excluded participants with a diagnosis of clinical depression or a debilitating illness. Depressed mood and overall wellbeing were assessed using standard questionnaires and weight was measured by trained nurses.

Of the 1,979 overweight and obese participants, 278 (14%) lost at least 5% of their initial body weight with a mean weight loss of 6.8kg per person. Before adjusting for serious health issues and major life events such as bereavement, which can cause both weight loss and depressed mood, the people who lost weight were 78% more likely to report depressed mood. After controlling for these, the increased odds of depressed mood remained significant at 52%.

“We do not want to discourage anyone from trying to lose weight, which has tremendous physical benefits, but people should not expect weight loss to instantly improve all aspects of life,” says lead author Dr Sarah Jackson (UCL Epidemiology & Public Health). “Aspirational advertising by diet brands may give people unrealistic expectations about weight loss. They often promise instant life improvements, which may not be borne out in reality for many people. People should be realistic about weight loss and be prepared for the challenges.

“Resisting the ever-present temptations of unhealthy food in modern society takes a mental toll, as it requires considerable willpower and may involve missing out on some enjoyable activities. Anyone who has ever been on a diet would understand how this could affect wellbeing. However, mood may improve once target weight is reached and the focus is on weight maintenance. Our data only covered a four year period so it would be interesting to see how mood changes once people settle into their lower weight.

“Healthcare professionals should monitor patients’ mental as well as physical health when recommending or responding to weight loss, and offer support where necessary. People who are trying to lose weight should be aware of the challenges and not be afraid to seek support, whether from friends, family or healthcare professionals.”

Senior author Professor Jane Wardle, director of the Cancer Research UK Health Behaviour Centre at UCL, says: “A recent UK survey found that 60% of overweight and obese adults in the UK are trying to lose weight. There are clear benefits in terms of physical health, which our study confirmed. People who lost weight achieved a reduction in blood pressure and serum triglycerides; significantly reducing the risk of heart disease. However, patients and doctors alike should be aware that there is no immediate psychological benefit and there may be an increased risk of depression.”

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